This invention relates to a method and apparatus for measuring an electric quantity by using an optical converter, and more particularly to a method and apparatus for measuring direct current, alternating current or high frequency current or voltage over a wide range by using an optical converter.
With enlargement of the scale of an electric power system, the electric power system becomes to have a large capacity and utilizes a ultra high voltage. For this reason, the information system such as protective relaying system, measuring system and operation control system can be controlled only by using electronic devices. In such a case, as a means for coupling the power system and the electronic devices and for converting the high voltage and the large current of the power system, a transformer or a capacitor has been used but there are such problems as the insulation of the interface, response speed and measuring range. Accordingly, converting apparatus have been used wherein high voltage or large current to be measured is supplied to an element manifesting an electro-optical effect or a magneto-optical effect for modulating light impinging upon a light sensitive element and detecting the variation in the quantity of the light as a measure of the high voltage or large current.
The principle of a current conversion type light converter will firstly be described with reference to FIGS. 1 and 2. The detail of this principle is described in H. Kobayashi's paper "Laser Application Technique" published on Nov. 25, 1975 by Nikkan Kogyo Shinbunsha, Tokyo.
As shown in FIG. 1, the current conversion type light converter comprises a source of laser light 11, a light pipe, for example, an optical fiber 12, a polarizer 13, a Faraday rotator comprising a solenoid coil 16 surrounding a lead glass rod 15, an analizer or polarizer 17, an optical fiber 18, a light receiver 19, an amplifier 20, an indicator 21, a focusing lens 22, and a source 23 for energizing the solenoid coil.
The coherent light emitted by the laser is converted into a linearly polarized parallel beam by the optical fiber 12 and the polarizer 13 and then enters into the Faraday rotator 14 as shown in vector diagram A where the light is rotated in polarization plane by an angle proportional to the magnetic field produced by the solenoid coil 16 by the input current I which is to be measured. The angle of rotation .theta. is given by the following equation ##EQU1## WHERE V represents the verdet's constant for FG-62 lead glass: 0.098min/gauss.cm, for example, l the length of the glass rod 15 or Faraday element and H.sub.s the magnetic field per unit area.
Since the magnetic field is a potential function, it can be expressed in terms of the difference in the magnetic potential U between the opposite ends of the Faraday element 15, the number of turns N of the solenoid coil N and current I as follows: ##EQU2##
The light rotated by the polarization plane passes through the analyzer 17 in proportion to the set angle thereof and then enters into the light receiver 19 through the optical fiber 18. As shown in vector diagrams A and B, when the analyzer 19 is set at an angle .theta..sub.0 with respect to vector P.sub.I, the output light P.sub.0 is expressed as follows. EQU P.sub.0 = P.sub.I cos (.theta..sub.0 - .theta.) (3)
Assuming now that the output of the light receiver 19 has a perfect square characteristic, the output of the light receiver V.sub.0 can be expressed by the following equation EQU V.sub.0 = K {cos(.theta..sub.0 - .theta.)}.sup.2 = K/2[1 + cos 2(.theta..sub.0 - .theta.)] (4)
where K represents the detector output expressed by EQU K = K.sub.D P.sub..gamma. T (mV)
where
K.sub.D : a detector constant (mV/mW) PA1 t : detector transmission coefficient (%) PA1 P.gamma. : detector incident light (mW) PA1 .theta..sub.0 = .pi./4, equation 4 becomes EQU V.sub.0 .apprxeq. K .theta. = KVNI (5) PA1 l : length of the element PA1 .lambda. : wavelength of light PA1 n.sub.0,n.sub.e : refraction coefficient of the ordinary and extraordinary ray under zero electric field PA1 .gamma..sub.ij : electro-optical coefficient of the element PA1 Ez : intensity of electric field applied to the element by the input voltage V of the source 41. Ez = Vzpp/d where Vzpp represents peak to peak AC voltage impressed upon the element. PA1 d : thickness of the Pockels element PA1 l : length of the Pockels element PA1 I.sub.0 : light input PA1 K.sub.p : transmittivity PA1 Ex : intensity of the electric field applied to the element by the input voltage V from source 41. Ex = Vxpp/d. PA1 Vxpp : peak to peak AC voltage impressed upon the element.
where
meaning that a small .theta. manifests a linear characteristic. The error E.sub.R caused by the deviation of the curve of sin 2.theta. from a straight line is expressed by the following equation. EQU E.sub.R 32 (2 .theta. - sin 2.theta.)/2 .theta. (6)
Angle .theta. = .pi./4 corresponds to the optimum operating point (central point) of an AC input characteristic shown in FIG. 2. At points other than this angle the linearlity is lost so that when the angle exceeds .pi./2, a large indication error is caused in the period. In this manner, when the width of the input I.sub.IN exceeds the position of .pi./2, a strain appears in the output V.sub.out.
From the foregoing description, it will be noted that there is a limit on the operating range of a current converter utilizing Faraday rotation.
Furthermore, a light converter shown in FIG. 3 has also been proposed wherein Pockels effect or Kerr effect is used. The principle of this type of the light converter is described in S. Saito et al paper in I.E.E.E., QE-2 page 225, Aug. 1966. The arrangement of various optical elements is similar to that shown in FIG. 1.
In the converter shown in FIG. 3, the light emitted by a laser 31 transmits through a light transmission path including an optical fiber 32, a Pockels effect element 36 such as for example LiNbO.sub.3 or quartz interposed between an orthogonally intersecting Nichols polarizer 33 and an analyzer 34, and a wavelength plate 37, and then through a focusing lens 38, and an optical fiber 39 to a light receiver 43. The Pockels effect element creates birefringence due to the voltage applied from a source 41. Accordingly, a phase difference is caused between the ordinary ray or light and the extraordinary ray or light of the linearly polarized light La thus producing an eliptically polarized light Lb. Further, the light is optically biased by the wavelength plate 37 and then enters into the analyzer 34. Only the component of the light corresponding to the set angle of the analyzer 34 passes to the light receiver 43 and then amplified by amplifier 44. The output of this amplifier is sent to an indicator 45 whereby an output voltage proportional to the voltage to be measured supplied from source 41 is indicated. The phase difference or optical retardation .delta. between the ordinary ray and the extraordinary ray of a transverse type Pockels effect element 36, is expressed by the following equation. EQU .delta. = 2.pi.l/.lambda. {(n.sub.0 - n.sub.e) + 1/2(.gamma..sub.33 n.sub.e.sup.3 - .gamma.13.sup.n 0.sup.3)Ez} (7)
where
Consequently, the output voltage Vpp of the light receiver 43 is expressed by the following equation EQU Vpp = 2.pi./2.lambda. (.gamma..sub.33 n.sub.e.sup.3 - .gamma..sub.13 .sub.0.sup.3)K.sub.T .multidot.K.sub.P .multidot.I.sub.0 .multidot.l/d.multidot.Vzpp (8)
where
In a Z bar type Pockels effect element, .delta. and Vpp are expressed by the following equations: EQU .delta. = 2.pi.l/.lambda. .gamma..sub.22 n.sub.0.sup.3 Ex (9) EQU Vpp = .pi./.lambda..multidot..gamma..sub.22 n.sub.0.sup.3 K.sub.T .multidot.K.sub.P .multidot.I.sub.0 .multidot.l/d.multidot.V.sub.xpp ( 10)
where
The input/output characteristic of the light converter of the type shown in FIG. 3 is shown by a curve L shown in FIG. 4. The range in which the amplitude of the output light V.sub.out corresponding to an input V.sub.in is the range in which the phase difference .delta. between the ordinary ray and the extraordinary ray is smaller than .pi. (that is, the range in which the amplitude of the input V.sub.in is smaller than V.sub..pi.) as can be noted from the characteristic curve. The voltage at the upper limit .delta. = .pi. is termed a "half wavelength voltage V.sub..pi. ".
For this reason, the conventional light converter of the type shown in FIG. 3 has generally been used in a thick line range l shown in FIG. 3 for detecting the amplitude of the output light. Since the voltage V.sub.in of the source 41 applied to the element 36 is set at the central operation point .pi./2, when the amplitude of the impressed voltage V.sub.in is larger than the half wavelength voltage V.sub..pi., due to nonlinearity of the characteristic L shown in FIG. 4, the output is limited thus disenabling the measurement. More particularly, when the amplitude of the input V.sub.in increases beyond the range indicated by the thick line l, the output V.sub.out is greatly distorted due to the non-linearity of the characteristic giving an unreliable measurement value.
As has been discussed hereinabove the voltage and/or current measuring range of the prior art light converters is limited to the linear range of the characteristics shown in FIGS. 2 and 4. Although it is possible to make the voltage or current to be measured to always coincide with the linear region by changing the tap of a transformer in combination with resistors or capacitors where the value of the voltage or current to be measured can be expected, such method is not only troublesome but also impossible to measure such abnormal voltage or current caused by a short circuit or lightening.